A series of peptides having the general folmulae, R1-R2-R3-Ser-Tyr-R6-R7-Arg-Pro-R10 will be prepared. Thirty different substitutions for R1 are considered and will allow an appreciation of the effects of size, shape, charge, aromaticity, and conformation of one position residues on the antagonist potencies of resultant peptides. The interaction of changes at R1, with selected modifications at R2 and R3 will also be examined. Several peptides with changes at R6 (other than D-Trp) and at R7 and R10 will be prepared, which incorporate the optimum modifications to the N-terminal three amino acids. Finally, a few miscellaneous peptides which are not part of these series are proposed.